Friday, July 17, 2015

A Change of Scene

A very welcome change in the weather pattern has developed in the past several days, with western and southern Alaska coming under increasing influence from low pressure in the Bering Sea and Aleutians. The contrast from the second half of June is great: see the 500 mb height anomaly maps for June 16-30 and July 1-15, below:

Significant rain has fallen or is now falling in parts of the western interior, with McGrath - highlighted here for its drought conditions just 3 days ago - recording 1.23" of rain in the past 3 days. More is to come in association with the current cyclonic system that is moving over western Alaska. The 500 mb height analysis below, courtesy of Environment Canada, shows the situation at 4am AKDT this morning.

Today's upper-level low is fairly strong for the time of year: Bethel observes a 500 mb height this low in July in fewer than 1 in 4 years (although interestingly it happened in each of 2011, 2012, and 2013). In terms of standardized anomaly, it is the most negative 500 mb height anomaly in more than 2 years in Bethel; in McGrath it's the most negative since February of 2014. The chart below shows the observed 500 mb height anomalies at McGrath since the beginning of 2014. Note the persistence of above-normal heights throughout last winter and in the past couple of months.

Here are some radar loops from Bethel today, showing the cyclonic circulation associated with the system. The animations show consecutive 2-hour periods.

Another symptom of the change is that - according to my data - nowhere in Alaska has exceeded 80 °F since July 10. Let's hope the new pattern sticks around: the state's fire acreage growth has completely stalled in the past few days.

3 comments:

Does the pattern indicate a reformation of the Pacific NW Blob I referenced in the preceding Blog? It appears a H is positioned over the NE Gulf of Alaska but I don's know it that's a new height formation or a repeat of an earlier pattern.